1. Field of the Invention
This invention relates to a disk-type single-phase brushless motor of a coreless structure which can start itself, and more particularly to a disk-type single-phase brushless motor improved in the noise problem, thereby significantly reducing noises such as vibrational noises and resonant sounds which occur when a conventional disk-type single-phase brushless motor of the same type is mounted as an axial-flow fan motor on a casing.
2. Description of the Prior Art
More and more fan motors have been being used for cooling electronic appliances of various types including office automation machines and devices, computer peripherals, power supply units and some other electronic appliances equipped with devices mounted at a high density (hereinafter called simply "high-density electronic appliances").
Particularly, axial-flow fan motors share most part of such fan motors.
This is because there is an increasing need for axial-flow fan motors arising from progressively increasing employment of electronic parts in various apparatus and systems and also from high-density mounting of such electronic parts and devices. Accordingly, it can be said that reduction in size or the size of an electronic appliance depends upon an axial-flow fan motor.
Of late, axial-flow fan motors are used very often due to the following reasons. The prices have dropped reduced significantly. It has also been recognized widely that an axial-flow fan motor is a means useful for the maintenance and improvement of the performance of an electronic appliance, above all, one of its parts.
In view of the current trend toward high-density mounting of devices, an electronic appliance should be designed to minimize heat generation and to withstand the surrounding temperature so that no air-cooling is required practically, in other words, no axial-flow fan motor is needed in order to achieve further size and cost reduction on the electronic appliance.
However, it is the existing state of things that, because such a trend of high-density mounting of devices requires further reduction in size and improvement in accuracy of an electronic part, it cannot be avoided to use an axial-flow fan motor for cooling such devices in order that heat may not have an influence upon the characteristics of the electronic part.
Here, the trend of high-density mounting of devices has now required further reduction in size and thickness as well as in weight comparing with conventional axial-flow fan motors which have been used so far.
The above requirement has arisen mostly for the following reasons. The high-density electronic appliance involves only a limited spacing in which an axial-flow fan motor is to be installed because it is designed strictly for reduction in size. The weight of an axial-flow fan motor is required to be so light that it can be supported easily by a housing or some other elements. More electronic appliances have been adopted, which can achieve their own functions with air-cooling of a lower degree because they are designed to cope with the problem of heat. There are more electronic appliances which work well without need for a large and expensive axial-flow fan motor of a very large wind volume. There are more electronic appliances in which other requirements such as small size, light weight, flat configuration and low cost have higher importance.
Meanwhile, such an instance often occurs that it is desired to incorporate an axial-flow fan motor for cooling in an electronic appliance from anxiety about generation of heat after the electronic appliance has been designed with a recognition that there is no necessity of cooling the same. Generally in the high-density electronic appliance, however, a conventional axial-flow fan motor of a considerable size cannot be installed because it is designed strictly with its spacing minimized and accordingly will involve no sufficient empty spacing as described above. Therefore, an axial-flow fan motor of a reduced size and thickness is required.
It is to be noted that axial-flow fan motors are divided into three typical types including an ac induction motor type, a dc brush motor type and a presently prevailing brushless (dc) motor type. Brushless motors are used very often for axial-flow fan motors due to many advantages thereof comparing with motors of the other types.
In recent years, brushless motors have been used widely due to an advantage that they are generally high in reliability in addition to characteristics thereof as a dc motor that they produce a high toque for their size and are good in controllability.
Further, disk-type brushless motors of an axial gap structure which have an axial air gap therein are widely used for disk-type brushless axial-flow fan motors for use with office automation machines and devices and the like because they can be readily reduced in thickness.
Here, a brushless motor has a drawback that it is high in cost because it requires, for each of a number of phases thereof, a position-detecting element such as a Hall effect element for detecting a north or south magnetic pole of a magnet rotor and a circuit for switchably energizing an armature coil or coils (i.e. a driving circuit which may otherwise be called electronic commutating circuit) using the position-detecting element.
Accordingly, it is not a good policy to use such an expensive brushless motor having a plurality of phases for an axial-flow fan motor which is used to blow a wind for cooling.
Therefore, for axial-flow fan motors, single-phase brushless motors are used which require only one position-detecting element and only one driving circuit for the single phase because it can be produced at a low cost.
However, such a single-phase brushless motor has a so-called dead point at an energization switching point at which the motor provides zero torque.
In order to eliminate such dead points to enable a single-phase brushless motor to start itself from any position of the rotor, normally a specific means for generating a torque is provided at each of such dead points of the motor. In particular, a reluctance torque generating member for generating a reluctance torque is provided at each of possible dead points of the motor at which no electromagnetic torque is generated by an armature coil or coils and a cooperating rotor including a field magnet.
In a coreless motor, the following methods are known by way of example for generating a reluctance torque. Referring first to FIG. 7, a rotor 2 in the form of a 6-pole field magnet having an alternate arrangement of the 6 north and south poles is mounted on a rotor yoke 1 in an opposing relationship to a stator yoke 5 with an air gap 4 left therebetween and with a pair of coreless armature coils 3 disposed in the air gap 4. In the motor of FIG. 7, the stator yoke 5 has at a face thereof opposing the rotor two inclined surfaces which thus define the complementarily inclined air gaps 4. This method, however, has a drawback that the efficiency is relatively low because the air gap is relatively great and the construction is complicated, resulting in increase in cost of the motor.
Referring now to FIG. 8, another method for generating a reluctance torque is illustrated. In the motor of FIG. 8, a pair of iron bars 6 are mounted on a stator yoke 5 and each extends through one of a pair of coreless armature coils 3 disposed in a uniform air gap 4 defined by the stator yoke 5 and a magnet rotor 2 on a rotor yoke 1. According to this arrangement, a magnetic flux 7 will appear as illustratively shown in FIG. 9 and hence the magnet rotor 2 will stop at a position where each of the iron bars 6 coincides with the center of one of the north and south poles of the magnet rotor 2. Accordingly, the armature coils 3 are located so as to produce a rotational torque at such stopping positions of the magnet rotor 2 and a position-detecting element 29 (FIG. 12) is located at a position at which it can detect one of the north and south poles of the magnet rotor 2 in order to enable the single-phase brushless motor of a coreless structure to start itself.
However, the method as shown in FIG. 8 has a drawback that if the thickness of the iron bars 6 is increased to increase the reluctance torque in order to ensure self-starting of the motor further, a phenomenon that the reluctance torque around the dead points decreases will appear because the magnetic flux will act as illustrated in FIG. 10 around the dead points to produce a magnetically stable condition.
It is to be noted that, in order to obtain an ideal torque - angular rotor-displacement curve, it is necessary to obtain a composite torque curve 8 as seen in FIG. 11. Referring to FIG. 11, a curve of an electromagnetic torque generated by an armature coil is indicated at 9 while a curve of a reluctance torque generated by a reluctance torque generating magnetic member such as the iron bar 6 mentioned above is indicated at 10.
As apparent from the electromagnetic torque curve 9 and the reluctance torque curve 10, the reluctance torque should preferably be one half of the armature torque in magnitude. This will provide the composite torque curve 8 indicative of a substantially uniform rotational torque over the entire range of rotation.
In order to obtain such an ideal composite torque as indicated by the curve 8, a reluctance torque generating magnetic member such as the iron bar 6 must be designed correctly in size and location.
Here, if the single-phase brushless motor of the principle illustrated in FIG. 7 or 8 does not include such an inclined air gap 4 or an iron bar 6 for generating a reluctance torque, such an electromagnetic torque curve 9 as shown in FIG. 11 will be obtained by the armature coil 3, but a dead point at which the motor produces no torque will appear at each energization switching point 11 of the motor. If the position-detecting element is positioned in an opposing relationship to one of such dead points 11 when the magnet rotor 2 is stopped, energization of the brushless motor will not cause the motor to generate a rotational torque. Accordingly, the motor is not suitable for practical use.
In view of such circumstances as described above, the inventors have already proposed a single-phase brushless motor of a coreless structure which attains such an ideal composite torque curve as the curve 8 shown in FIG. 11 to enable self-starting of the motor and only involves certain improvement in a stator yoke to ensure self-starting of the motor further without necessitating a special reluctance torque generating means.
A disk-type single-phase brushless axial-flow fan motor for which such a proposed single-phase brushless motor of a coreless structure is employed will be described below with reference to FIGS. 12 to 16. The fan motor generally designated at 12 includes a body 13 including an outer casing 14 which has a substantially square shape in plan and has a hollow spacing or bore formed at a central portion thereof as shown in FIG. 13. The motor body 13 further includes a generally cup-shaped motor casing 16 located in the center bore of the outer casing 14 and connected to the outer casing 14 by a plurality of radially extending stays 15 (only one is shown in FIG. 13). The motor casing 16 has a substantially channel-shaped vertical section and defines a motor receiving spacing therein. The casings 14 and 16 and stays 15 are formed as a unitary member from plastic material and define thereamong a plurality of perforations 18 through which air flows produced by impellers 17 which will be hereinafter described pass.
A pair of support posts 19 are formed uprightly in an integral relationship on the bottom of the motor casing 16 with a plastic material. A printed circuit board 20 is mounted at the top ends of the support posts 19, and a stator yoke 21 is located on the printed circuit board 20. A pair of coreless armature coils 22 and 23 are mounted in an antipodal relation at the same-phase positions on the upper face of the stator yoke 21 as shown in FIG. 14, thereby forming a coreless stator armature 24. The stator armature 24 is secured to the top ends of the support posts 19 by means of a pair of fastening screws 25 (only one is shown in FIG. 12) of non-magnetic material which extend through the stator yoke 21 and the printed circuit board 20 and are threaded into threaded holes formed in the support posts 19.
Here, in order to minimize the cost, the stator yoke 21 is required to additionally function as a reluctance torque generating member for enabling the disk-type single-phase brushless axial-flow fan motor 12 to start itself as described hereinabove.
It is to be noted that, as described hereinabove, generally a single-phase brushless motor has a drawback that, if it is stopped at a position in which its rotor and position-detecting element are positioned at a dead point, energization thereof will not cause self-starting of the motor without the provision of an additional self-starting enabling means due to its single-phase energization construction. However, the reason why a single-phase brushless motor which has the drawback described just above is used for a disk-type single-phase brushless axial-flow fan motor is that it can be constructed at a low cost because it requires only one position-detecting element and only one driving circuit. To the contrary, in the case of a brushless axial-flow fan motor of a two- or three-phase construction which can start itself without an additional self-starting enabling means and is superior in efficiency and performance, driving circuits and position-detecting elements are required by a number corresponding to the number of phases of the motor, which requires a correspondingly high cost. Accordingly, it is not preferable to use such a poly-phase brushless motor as an axial-flow fan motor which must be produced at a minimized cost.
In this manner, a single-phase brushless motor structure must be employed for an ordinary brushless axial-flow fan motor, but a single-phase brushless motor has a drawback that it cannot start itself unless it has an additional self-starting enabling means.
An exemplary one of such self-starting enabling means for a single-phase brushless motor is additional provision of a reluctance torque generating member. The provision of a reluctance torque generating member will, however, raise the cost of a motor accordingly. The axial-flow fan motor 12 thus employs a specifically devised structure wherein the stator yoke 21 has an additional function as a reluctance torque generating member.
In particular, referring to FIG. 14, the stator yoke 21 has a pair of cutaway portions 26 and 27 formed therein in such a configuration as to enable self-starting of the single-phase brushless axial-flow fan motor 12. It is to be noted here that the mere formation of a cutaway portion or cutaway portions in the stator yoke 21 could not assure optimum self-starting of the axial-flow fan motor 12. Theoretically most desired shapes and locations of the cutaway portions 26 and 27 will next be described.
The pair of cutaway portions 26 and 27 are formed at symmetrical positions spaced circumferentially by an angle of 180 degrees from each other in the stator yoke 21 located on the motor casing 16. The cutaway portions 26 and 27 have an angular or circumferential width equal to a mechanical angle of about 90 degrees. The specific configuration of the stator yoke 21 has originated from the fact that a magnet rotor 28, which will be hereinafter described with reference to FIG. 16, has up to 4 alternate north and south poles each having an angular or circumferential width of about 90 degrees.
With the stator yoke 21 in which such cutaway portions 26 and 27 are formed, opposite radially extending edges 26A, 26B and 27A, 27B defining the cutaway portions 26 and 27, respectively, present magnetically neutral positions with respect to the magnet rotor 28 at one of which the magnet rotor 28 will be stopped when the axial-flow fan motor 12 is deenergized. If the armature coils 22 and 23 are energized in the stopped position of the magnet rotor 28, the brushless axial-flow fan motor 12 will start itself without fail. Accordingly, if the coreless armature coils 22 and 23 are energized in respective predetermined directions in response to a signal from a position-detecting element, then the magnet rotor 28 shown in FIG. 16 will be rotated in the direction indicated by an arrow mark A (also referred to FIG. 14) without fail by a rotational torque generated in accordance with the Fleming's left-hand rule. It is thus necessary to dispose the two armature coils 22 and 23 at such locations as seen in FIG. 14 on the stator yoke 21 as to allow self-starting of the motor and also to dispose a single position-detecting element 29 (refer to FIG. 12) similarly at such a location.
Referring to FIG. 14, it is only magnetically-active radial conductor portions 22a, 22b and 23a, 23b of the armature coils 22 and 23 that contribute to the generation of a rotational torque. Circumferentially-extending conductor portions 22c, 22d and 23c, 23d do not contribute to the generation of a rotational torque.
In order to attain self-starting of the axial-flow fan motor 12 in a desired manner by the cutaway portions 26 and 27 of the stator yoke 21 and the armature coils 22 and 23, it is most preferable to form the cutaway portions 26 and 27 such that they have an angular width substantially equal to n.multidot..theta. where n is an integer equal to or greater than 1 and equal to or smaller than the number of the magnet poles of the magnet rotor 28 and .theta. is an angular width of each pole of the magnet rotor 28 and also to position the stator yoke 21 relative to the armature coils 22 and 23 such that the opposite radially extending edges 26A, 26B and 27A, 27B of the cutaway portions 26 and 27 are located either at positions spaced by an angular distance of about n.multidot..theta./4 degrees in the direction opposite the direction of rotation of the magnet rotor 28 (in the direction opposite the direction of the arrow mark A) from the positions of the center lines of the magnetically active conductor portions 22a, 22b and 23a, 23b of the armature coils 22 and 23 or at the same-phase positions (which coincide, in the arrangement of FIG. 14, with the positions of the magnetically active conductor portions 22a, 22b and 23a, 23b because the magnet rotor 28 has just four magnet poles) at each of which a maximum starting torque can be obtained.
In the arrangement shown in FIG. 14, the stator yoke 21 is positioned such that the opposite radial edges 26A, 26B and 27A, 27B of the cutaway portions 26 and 27 thereof are located at positions spaced by an angular or circumferential distance equal to one fourth of each circumferential pole width, that is, by a mechanical angle of 22.5 degrees in the direction opposite the direction of rotation of the magnet rotor 28 (in the direction opposite the direction of the arrow mark A) from the respective magnetically active conductor portions 22a, 22b and 23a, 23b of the armature coils 22 and 23 at the respective positions of which the magnetically active conductor portions 22a, 22b and 23a, 23b provide a maximum starting torque. With the arrangement, the single-phase brushless axial-flow fan motor 12 can start itself in a desired manner without employing an additional self-starting enabling means which will particularly raise the cost of the motor 12 accordingly.
Referring back to FIG. 12, electronic parts 41 in the form of chips constituting a driving circuit are mounted on a lower face of the printed circuit board 20 located on a lower face of the stator yoke 21 while the two coreless armature coils 22 and 23 and the single position-detecting element 29 such as a Hall effect element shown in FIG. 12 are mounted on an upper face of the stator yoke 21. The position-detecting element 29 is located at a position opposing one of the magnetically active conductor portions 22a, 22b and 23a, 23b of the armature coils 22 and 23 or at one of the same-phase positions. The armature coils 22 and 23 and the position-detecting element 29 constitute the coreless stator armature 24 and are disposed for relative rotation in a face-to-face opposing relationship to the magnet rotor 28 with an axial air gap left therebetween.
It is to be noted that since the axial-flow fan motor 12 of the arrangement shown has 4 magnet poles as illustratively shown in FIG. 16, it is necessary to locate the position-detecting element 29 at a position opposing one of the magnetically active conductor portions 22a, 22b and 23a, 23b of the armature coils 22 and 23. However, since provision of the position-detecting element 29 at a position on an upper face of one of the magnetically active conductor portions 22a, 22b and 23a, 23b opposing the magnet rotor 28 will increase the distance of the air gap (field air gap) by a distance equal to the thickness of the position-detecting element 29, which will make the field magnet flux weak accordingly, the disk-type single-phase brushless motor will be deteriorated in that a high torque cannot be obtained and the efficiency is low. Therefore, in the present arrangement, the single position-detecting element 29 is located at a position on the upper face of the printed circuit board 20 opposing one of the magnetically active conductor portions 22b and 23b of the armature coils 22 and 23 which do not oppose the stator yoke 21, that is, which oppose the cutaway portions 26 and 27 of the stator yoke 21. Accordingly, the position-detecting element 29 is typically located at either one of two positions indicated by broken line circles 30 and 31 in FIG. 14, and at the position, it is located between the upper face of the printed circuit board 20 and a lower face of the magnetically active conductor portion 23b or 22b of the armature coil 23 or 22 in the cutaway portion 30 or 31 of the stator yoke 21.
The magnet rotor 28 includes a field magnet having up to 2P poles (P is an integer greater than 1) wherein the north and south magnetic poles are formed in an alternate relationship with the same circumferential width of an angle of 90 degrees as illustrated in FIGS. 15 and 16, and in the arrangement shown, the magnet rotor 28 has a flattened annular profile with 4 magnetic poles. Referring to FIG. 12, the magnet rotor 28 is formed in an integral relationship with a cup-shaped body 34 of a rotary fan 33 particularly shown in FIG. 15 made of plastic material by molding with a rotor yoke 32 interposed therebetween. A plurality of impellers 17 are formed in an integral relationship on an outer circumferential periphery of the cup-shaped body 34, and when the cup-shaped body 34 is rotated, the impellers 17 will produce and send cooling air flows toward the perforations 18 below.
A hub 35 is formed at a substantially central portion of a lower face of the cup-shaped body 34, and an upper end portion of a rotary shaft 36 is secured to the hub 35 for integral rotation with the rotary fan 33.
The rotary shaft 36 is supported for rotation by means of a pair of ball bearings 38 and 39 mounted in a pair of openings at upper and lower ends of a bearing housing 37 which is formed uprightly in an integral relationship at a central portion of the motor casing 16.
A retaining ring 40 is mounted at a portion of the rotary shaft 36 spaced a little distance below the lower ball bearing 39 so that the shaft 36 may not be pulled off upwardly.
The disk-shaped single-phase brushless axial-flow fan motor 12 is useful indeed and has been reduced to practice by the assignee of the present patent application.
However, here has arisen a problem. In particular, the problem has been caused by the fact that, as such axial-flow brushless fan motors as described above have been considered as an important component of the high-density electronic appliance and has been used progressively frequently, very severe requirements have been required that, depending upon a type of the high-density electronic appliance, an axial-flow brushless fan motor be further reduced in size and/or thickness and in cost and production of noises caused by vibrational noises and resonant sounds be reduced extremely.
In the high-density electronic appliance, a large number of parts or devices which are made of various materials are installed generally within a limited area or volume. Accordingly, if a conventional disk-type single-phase brushless axial-flow fan motor incorporated in such a high-density electronic appliance is driven, depending upon a type of the high-density electronic appliance, very high noises caused by vibrational noises and resonant sounds are produced in some cases.
The conventional disk-type single-phase brushless axial-flow fan motor 12 described with reference to FIGS. 12 to 16 has been devised such that it may rotate relatively smoothly and accordingly it may not produce high turning noises nor vibrational noises while it has a single-phase energization structure, and besides a desired reluctance torque and accordingly a desired characteristic of the motor may be obtained.
The disk-type single-phase brushless axial-flow fan motor 12 does not produce such high vibrations as can be felt by a hand or in other words produces very low vibrations, and produces only very low turning noises which may or may not be caught by an ear. Accordingly, there may be little problem if the disk-type single-phase brushless axial-flow fan motor 12 is employed in an ordinary high-density electronic appliance, and in such a case, noises of vibrational noises and resonant sounds can be almost ignored. However, where such a disk-type single-phase brushless axial-flow fan motor is mounted on a casing of the high-density electronic appliance of a certain type as described above, it is a problem that high noises may sometimes be produced because vibrations of the motor are transmitted with an amplifying effect by the casing to develop high vibrating and resonant sounds.
Here, reduction in high noises of vibrational noises and resonant sounds of such a disk-type single-phase brushless axial-flow fan motor can be attained by a method disclosed in U.S. Pat. No. 4,620,139 or by another method disclosed in Japanese Utility Model Laid-Open No. 62-104565 both assigned to the applicant of the present patent application wherein one of a pair of radially extending edges of a cutaway portion of a stator yoke is inclined in a skew. However, in the conventional method, the relatively opposing area of a boundary portion of a north pole and an adjacent south pole of a magnet rotor to the radially extending edge of the cutaway portion of the stator yoke is so small that self-starting or continued rotation of the motor may not be assured depending upon the magnitude or variation of the load. This often occurs particularly where the method disclosed in U.S. Pat. No. 4,620,139 is employed, and accordingly the method has a drawback that it is low in reliability. It is to be noted that the utility model application of Japanese Utility Model Laid-Open No. 62-104565 is a basic application made for the same object with the invention of the present patent application. The invention of the utility model application is very effective in that since one of a pair of radially extending edges of a cutaway portion formed in a stator yoke is formed in a skew while the other edge extends in a radial direction, a relatively high reluctance torque can be obtained, and besides since the reluctance torque is generated in such a manner as to increase gradually comparing with the arrangement with the stator yoke 21 shown in FIG. 14 due to the presence of the edge of the cutaway portion formed in a skew, the motor can rotate smoothly and production of noises is minimized.
However, a number of high-density electronic appliances involve a member or members which are ready to resonate, and even where a disk-type single-phase brushless axial-flow fan motor is mounted on such a readily resonating member, it is further required to minimize noises caused by vibrational noises and/or resonant sounds in the motor wherein such a conventional stator yoke as disclosed in Japanese Utility Model Laid-Open No. 62-104565 is employed.
Thus, the inventors made a stator yoke wherein opposite radially extending edges of a cutaway portion therein are formed each in a skew. However, indeed the arrangement decreased production of high noises caused by vibrational noises and/or resonant sounds, but it presented a drawback that the reluctance torque is so low that, if a somewhat high load is applied to the motor by some external factor, the starting characteristic is deteriorated and in some cases self-starting and/or continued rotation is disabled.
Then, the inventors made further examination of the method of Japanese Utility Model Laid-Open No. 62-104565 in order to attain optimum designing and produced and examined several types of stator yokes wherein only one of a pair of radially extending edges of a cutaway portion is formed in a skew in different angles. However, a satisfactory result was not reached in regard to reduction in noises caused by resonant sounds and vibrational noises nor in regard to the magnitude of the reluctance torque obtained.
It has been found, however, that the shape of a cutaway portion of a stator yoke has a considerable effect on the characteristics of the motor although the matter is delicate.